The isomorphous substitution of various elements into the framework of zeolites has received considerable attention in recent years, as such substitution offers interesting opportunities for preparing materials with novel catalytic properties. One such material is titanium silicalite-1 (generally known as TS-1), which is a titanosilicate having an MFI-type structure. Titanium silicalite has been found to be particularly useful in oxidation reactions involving hydrogen peroxide, such as the epoxidation of olefins to yield epoxides. See, for example, U.S. Pat. No. 4,833,260.
However, it has proven to be exceedingly difficult to prepare TS-1 containing a relatively high concentration of Ti atoms in its framework structure. The research group which first prepared TS-1 has reported in numerous publications that it is not possible to incorporate more than 2.5 mole % TiO.sub.2 into the TS-1 framework. That is, calcined TS-1 has the molar composition xTiO.sub.2.(1-x)SiO.sub.2, with x ranging from close to 0 to a maximum of 0.025. When a large amount of titanium reagent is utilized in the typical hydrothermal direct synthesis method used to prepare TS-1, the precipitation of extra framework phases (e.g., TiO.sub.2 anatase and/or an amorphous titania phase) is observed. See, for example, Millini et al., Gazzetta Chemica Italiana 126, 133-140 (1996).
However, other workers have claimed that TS-1 materials containing greater than 2.5 mole % framework titanium may be produced by varying the type of silicon and/or titanium reagent used in the hydrothermal procedure to provide a better match between the hydrolysis rates of these reagents. See, for example, Thangaraj et al., J. Catalysis 130, 1 (1991) and Tuel et al., Appl. Catal. 110, 137 (1994). Such claims have stimulated an on-going debate over the amount of framework titanium actually present in such "titanium-rich" silicalites and the existence of different titanium sites.
Our investigations of the catalytic performance of the "titanium-rich" silicalites described in the literature have found that certain of these substances (contrary to the predictions of prior art investigators) are capable of providing high epoxide selectivity. See U.S. Pat. No. 5,262,550. However, we have also found that the activity of such materials does not linearly increase as the total titanium content is increased beyond 2.5 mole % TiO.sub.2. This suggests that, regardless of whether all the additional titanium atoms being introduced actually are within the lattice framework of the zeolite, not all the titanium sites are equally active catalytically. Generally speaking, it will be desirable to use a catalyst having the maximum activity possible (as measured by moles of reactants converted per unit of time for a given weight of catalyst). The presence of titanium atoms which are unavailable to function as active catalyst sites or which convert the reactants at a slower rate than other sites results in a titanium silicalite having less than optimum catalytic activity.
Secondary routes for preparing TS-1 titanium silicalite have also been investigated, wherein a pre-synthesized MFI-type zeolite is treated with a volatile or water-soluble titanium compound. For example, an acid-leached ZSM-5 may be reacted with gaseous titanium tetrachloride (see Kraushaar et al., Catal. Lett. 1, 81 (1988), Kraushaar-Czarnetzki et al., Catal. Lett. 2, 43 (1989), and Huybrechts et al., Catal. Lett. 8, 237 (1991)). Unfortunately, the concentration of titanium incorporated by known secondary synthesis materials is even lower than that obtained by hydrothermal procedures. Attempts to increase the amount of titanium using secondary synthesis techniques similarly leads to the production of extra framework titanium species. Moreover, according to the previously mentioned Huybrechts et al. article, materials made by such methods are very poor olefin epoxidation catalysts when compared to titanium silicalites prepared by direct synthesis.